(a) Field of the Invention
The present invention relates to a wireless communication system and method for carrying out effective ranging, for the purpose of solving problems caused by inaccurate synchronization between subscriber stations in an uplink. More specifically, the present invention relates to a system for timing synchronization of an uplink between a subscriber and a base station in a multiple access wireless communication system using OFDMA.
(b) Description of the Related Art
The wireless access standard applied to IEEE 802.16a Wireless MAN-OFDMA PHY layer defines time division access in a downlink from a base station to a subscriber, and an uplink from the subscriber to the base station.
In orthogonal frequency division multiple access (OFDMA), each subscriber transmits data to an uplink through a sub-channel allocated thereto. Here, the subscriber should be connected with a base station during a given time slot for the uplink access. This is because when data is transmitted with sub-carriers allocated to subscribers, orthogonality among the sub-carriers is lost if timing synchronization among the subscribers is not achieved, resulting in interference of data of the subscribers. Accordingly, synchronization in the uplink is important in OFDMA.
To solve the above-described problem, the IEEE 802.16a Wireless MAN-OFDMA standard proposed a ranging method that obtains a timing delay error in a time domain using a pseudo random binary sequence (PRBS) in a frequency domain. Ranging means that a base station estimates a delay distance using a detected PRBS when a subscriber transmits the PRBS to the base station. IEEE 802.16a also proposed methods of demodulating a time domain signal using Fast Fourier Transformer (FFT) and then using Inverse Fourier Transform for ranging.
OFDMA carries out Inverse Fast Fourier Transform (IFFT) in order to modulate data generated in the frequency domain into a time domain, and performs FFT in order to demodulate the time domain signal into the data. Accordingly, a base station demodulator requires an additional IFFT in addition to FFT in order to estimate a delay in the time domain and a distance delayed in the time domain while the delay is detected in the frequency domain. This increases complexity of the configuration of the base station demodulator, and decreases processing speed due to multiple access.
If there is no additional IFFT process, calculations should be carried out in consideration of delay components in various cases. This requires complicated hardware, and the base station demodulator having this configuration is difficult to construct.